1. Field of the Invention
The present invention relates to a method of producing a polymer optical waveguide, and, particularly, to a polymer optical waveguide having a thin flexible filmy form.
2. Description of the Related Art
Among methods of producing a polymer waveguide, the following methods have been proposed: (1) a method in which a film is impregnated with a monomer, the core part is selectively exposed to light to change the refractive index and the film is then applied (a selective polymerization method), (2) a method in which a core layer and a clad layer are applied, and a clad part is then formed by using reactive ion etching (the RIE method), (3) a type of photolithographic method in which an ultraviolet ray-curable resin obtained by adding a light-sensitive material to a polymer material is used to carry out exposure and developing (a direct exposure method), (4) a method using injection molding and (5) a method in which a core layer and a clad layer are applied, and the core part is then exposed to light to change the refractive index of the core part (a photo-bleaching method).
However, in the selective polymerization method (1) a problem exists in the application of the film, methods (2) and (3) are expensive because a photolithographic method is entailed and in method (4) a problem exists surrounding the accuracy of a core diameter. Furthermore, method (5) also entails a problem insofar that a sufficient difference cannot be obtained between the refractive indices of the core layer and the clad layer.
At present only methods (2) and (3) are practical methods according to which it is possible to achieve a high level of performance. However, as mentioned above, both of these methods give rise to a problem of costs. Moreover, not one of methods (1) to (5) is appropriate for forming a polymer optical waveguide on a flexible plastic base material covering large area.
Another method of producing a polymer optical waveguide is known in which a polymer precursor material for the core is introduced into a patterned substrate (clad) formed into a groove pattern which is to become a capillary. The polymer precursor material is then cured to form a core layer and a plane substrate (clad) is applied to the surface of the core layer. However, when this method has been used, difficulties have occurred insofar that the polymer precursor material is introduced not only into the capillary groove but also spreads thinly over the entire space between the pattern substrate and the plane substrate. In consequence, when the polymer precursor material applied between both substrates is cured, a thin layer having the same composition as the core layer is formed, resulting in the leaking of light through the thin layer.
As one of the methods for solving this problem, David Heard has proposed a method in which a patterned substrate formed with a groove pattern which is to become a capillary is secured to a plane substrate by using a clamp jig, the contact part between the pattern substrate and the plane substrate is further sealed with a resin and pressure is then reduced. A monomer (diallyl isophthalate) solution is then introduced into the capillary to produce a polymer optical waveguide (see the specification of patent publication No. 3151364). According to this method, by using a monomer in place of a polymer precursor material as the core-forming resin material thereby lowering the viscosity of the filler material, and by introducing the monomer by means of a capillary phenomenon, the monomer is prevented from being introduced into any area other than into the capillary.
However, since according to this method a monomer is used as the core-forming material, a problem arises insofar that when the monomer is polymerized into a polymer, the volume shrinkage factor is substantial and transmission loss of the polymer optical waveguide is magnified. Moreover, this method is so complicated that the patterned substrate needs to be secured to the plane substrate by a clamp and in addition, the contact part has to be sealed with a resin. This process is therefore unsuitable for mass-production, and in consequence no reduction in costs can be expected. Also, it is impossible to apply this method to the production of a polymer optical waveguide using as the clad a film having a thickness of several mm or 1 mm or less.
Also, George M. Whitesides and others, in Harvard University have recently proposed, as one of the soft lithographic methods in new technologies making a nano-structure, a method called a capillary micro-mold. According to this method a master substrate is made using photolithography, the nano-structure of the master substrate is copied exactly onto a mold of a polydimethylsiloxane (PDMS) by taking advantage of the adhesiveness and ready releasability of the PDMS, and a liquid polymer is then introduced into the mold by utilizing a capillary phenomenon and secured. A detailed explanatory report appears in SCIENTIFIC AMERICAN SEPTEMBER 2001 (Nikkei Science, December issue (2001).
Also, Kim Enoch and others in the group of George M. Whitesides at Harvard University disclose a capillary micro-mold method (see U.S. Pat. No. 6,355,198). However, even if the production method described in this patent were applied to the production of a polymer optical waveguide, because the core part of the optical waveguide has a small sectional area, considerable time would be required to form the core part, thus rendering the method unfit for mass-production. There is also another drawback with this method insofar that a change in volume is caused when the monomer solution is polymerized to form a polymer, resulting in a change in the shape of the core and accordingly a substantial loss in transmission.
Moreover, B. Michel and others in the IBM Züfrich Research Center have proposed lithographic technologies having a high level of resolution obtained by using a PDMS. Reports suggest that a resolution of several tens of nm has been obtained. A detailed explanatory report appears in the SEPTEMBER 2001 issue of IBM J. REV. & DEV. (Vol. 45 No. 5).
As mentioned above, soft lithographic technologies using a PDMS and a capillary micro-mold method are nano-technologies on which many countries, in particular the USA, have been focusing.
However, if an optical waveguide were manufactured according to the kind of micro-mold method mentioned above, it would be impossible to make the volume shrinkage factor smaller during curing (and thus reduce transmission loss), and at the same time reduce the viscosity of a filler liquid (monomer or the like) to make introduction easy. Therefore, if priority is to be given to reducing transmission loss, it is not possible to maintain the viscosity of the filler liquid within a certain limit. This results in a low speed of introduction and mass-production can not be expected. Moreover, the aforementioned micro-mold method is based on the premise that a glass or silicon substrate will be used as the substrate and does not take into account the use of the kind of flexible film base material used in this method.
Meanwhile, JP-A No. 2002-311273 discloses a method for producing a polymer optical waveguide by using a mold having low rigidity. According to this method, a second convex mold is made from a first concave mold; a resin is applied to the second concave mold and cured to form a first clad having a concave portion which is to be a core pattern; a resin is applied to the concave portion which is to be a core pattern and after the second convex mold has been peeled off the resin is cured to form a core; and a resin is then further applied and cured to form a second clad. It is however difficult to introduce a core resin into only the concave portion and it is therefore difficult to manufacture a fine core pattern with a high degree of accuracy.
In the meantime, in current integrated circuit (IC) technologies and large scale integration (LSI) technologies, much attention is being focused on the fact that optical wiring is being used, in place of highly densified electric wiring between devices, between boards in devices and within chips, in order to quicken operational speed and to enhance the degree of integration.
As elements used for optical wiring, for example, optical fiber wiring boards have been put to practical use in which optical fibers are wired on a sheet board. However, in the case of optical fibers, the end surfaces of the optical fiber must be abraded and optical connectors for connection purposes are also expensive.
For the above reasons, demands have been made to decrease costs by using a polymer waveguide in place of an optical fiber. Also, it is desired to make a flexible polymer optical waveguide by taking advantage of the characteristics of a film. However, the film must usually be made thin to obtain a flexible waveguide. However, if the film is thin, its handling is difficult and a semiconductor process, a highly accurate processing method, cannot be used.
To cope with this demand, the inventors of the invention have proposed a method of producing a flexible polymer optical waveguide by a very simple method disclosed in JP-A No. 2004-86144 and the like. According to this method a mold is provided with a concave portion corresponding to a core, wherein a mold is brought into close contact with a clad film base material which is to be a lower clad layer, and a core-forming curable resin is introduced into the concave portion of the mold. By mean of this method a polymer optical waveguide can be provided at a very low cost with a reduction in propagation loss even though it is substantially less expensive than conventional methods of producing a polymer optical waveguide. However, this polymer optical waveguide still leaves further room for improvements in the reduction of propagation loss.